When repairing a hole in a piece of sheet metal in an automobile body, the metal all along the periphery of the hole is "indented" inwardly, to form a short shelf all around the hole. A flat piece of metal is then attached to the shelf with blind, or "pop" rivets. The depressed section thus created is then filled in with putty to slightly above the level of the original surface, sanded smooth, and painted.
It can readily be seen that the job of riveting the piece of metal to the shelf will be made easier if the shelf occupies a flat plane more or less parallel to the original surface.